1. Field of the Invention
The present invention relates to the use of liquid crystal materials for marking and authenticating documents. The invention is particularly suited for, but not limited to, use with documents subject to counterfeit such as government certificates, notes, and currency, stock certificates, bank notes and the like.
2. Description of the Prior Art
Liquid crystal materials have found widespread application in eye-readable displays as are found, for example, in digital watches, calculators, and the like. Considerable effort has been expended to develop liquid crystal materials which exhibit a high level of visual contrast. To this end, guest dyes have been linked to host liquid crystals to tailor the absorption or transmittance properties of the liquid crystal. See "High Order Parameter Anthraquinone Dye", A. Pearson and I. C. Sage, Chemical Development Research Department, B. D. H. Chemicals Ltd., Poole Dorset, England, June, 1981. In addition, it is known that liquid crystal materials can be protected by encapsulation. For example, see U.S. Pat. No. 3,585,381 teaching use of encapsulated liquid crystal materials in thermal imaging and other display applications.